DESCRIPTION

The pthread_mutex_destroy() function shall destroy the mutex
object referenced by mutex; the mutex object becomes,
in effect, uninitialized. An implementation may cause pthread_mutex_destroy()
to set the object referenced by mutex
to an invalid value. A destroyed mutex object can be reinitialized
using pthread_mutex_init(); the results of otherwise
referencing the object after it has been destroyed are undefined.

It shall be safe to destroy an initialized mutex that is unlocked.
Attempting to destroy a locked mutex results in undefined
behavior.

The pthread_mutex_init() function shall initialize the mutex
referenced by mutex with attributes specified by
attr. If attr is NULL, the default mutex attributes are
used; the effect shall be the same as passing the address of
a default mutex attributes object. Upon successful initialization,
the state of the mutex becomes initialized and unlocked.

Only mutex itself may be used for performing synchronization.
The result of referring to copies of mutex in calls
to pthread_mutex_lock(), pthread_mutex_trylock(), pthread_mutex_unlock(),
and pthread_mutex_destroy() is undefined.

In cases where default mutex attributes are appropriate, the macro
PTHREAD_MUTEX_INITIALIZER can be used to initialize mutexes
that are statically allocated. The effect shall be equivalent to dynamic
initialization by a call to pthread_mutex_init()
with parameter attr specified as NULL, except that no error
checks are performed.

RETURN VALUE

If successful, the pthread_mutex_destroy() and pthread_mutex_init()
functions shall return zero; otherwise, an
error number shall be returned to indicate the error.

The [EBUSY] and [EINVAL] error checks, if implemented, act as if they
were performed immediately at the beginning of processing
for the function and shall cause an error return prior to modifying
the state of the mutex specified by mutex.

ERRORS

The pthread_mutex_destroy() function may fail if:

EBUSY

The implementation has detected an attempt to destroy the object referenced
by mutex while it is locked or referenced
(for example, while being used in a pthread_cond_timedwait()
or pthread_cond_wait()) by another thread.

EINVAL

The value specified by mutex is invalid.

The pthread_mutex_init() function shall fail if:

EAGAIN

The system lacked the necessary resources (other than memory) to initialize
another mutex.

ENOMEM

Insufficient memory exists to initialize the mutex.

EPERM

The caller does not have the privilege to perform the operation.

The pthread_mutex_init() function may fail if:

EBUSY

The implementation has detected an attempt to reinitialize the object
referenced by mutex, a previously initialized, but
not yet destroyed, mutex.

EINVAL

The value specified by attr is invalid.

These functions shall not return an error code of [EINTR].

The following sections are informative.

EXAMPLES

None.

APPLICATION USAGE

None.

RATIONALE

Alternate Implementations Possible

This volume of IEEE Std 1003.1-2001 supports several alternative
implementations of mutexes. An implementation may
store the lock directly in the object of type pthread_mutex_t.
Alternatively, an implementation may store the lock in the
heap and merely store a pointer, handle, or unique ID in the mutex
object. Either implementation has advantages or may be required
on certain hardware configurations. So that portable code can be written
that is invariant to this choice, this volume of
IEEE Std 1003.1-2001 does not define assignment or equality for
this type, and it uses the term "initialize" to
reinforce the (more restrictive) notion that the lock may actually
reside in the mutex object itself.

Note that this precludes an over-specification of the type of the
mutex or condition variable and motivates the opaqueness of
the type.

An implementation is permitted, but not required, to have pthread_mutex_destroy()
store an illegal value into the mutex.
This may help detect erroneous programs that try to lock (or otherwise
reference) a mutex that has already been destroyed.

Tradeoff Between Error Checks and Performance Supported

Many of the error checks were made optional in order to let implementations
trade off performance versus degree of error
checking according to the needs of their specific applications and
execution environment. As a general rule, errors or conditions
caused by the system (such as insufficient memory) always need to
be reported, but errors due to an erroneously coded application
(such as failing to provide adequate synchronization to prevent a
mutex from being deleted while in use) are made optional.

A wide range of implementations is thus made possible. For example,
an implementation intended for application debugging may
implement all of the error checks, but an implementation running a
single, provably correct application under very tight
performance constraints in an embedded computer might implement minimal
checks. An implementation might even be provided in two
versions, similar to the options that compilers provide: a full-checking,
but slower version; and a limited-checking, but faster
version. To forbid this optionality would be a disservice to users.

By carefully limiting the use of "undefined behavior" only to things
that an erroneous (badly coded) application might do, and
by defining that resource-not-available errors are mandatory, this
volume of IEEE Std 1003.1-2001 ensures that a
fully-conforming application is portable across the full range of
implementations, while not forcing all implementations to add
overhead to check for numerous things that a correct program never
does.

Why No Limits are Defined

Defining symbols for the maximum number of mutexes and condition variables
was considered but rejected because the number of
these objects may change dynamically. Furthermore, many implementations
place these objects into application memory; thus, there is
no explicit maximum.

Static Initializers for Mutexes and Condition Variables

Providing for static initialization of statically allocated synchronization
objects allows modules with private static
synchronization variables to avoid runtime initialization tests and
overhead. Furthermore, it simplifies the coding of
self-initializing modules. Such modules are common in C libraries,
where for various reasons the design calls for
self-initialization instead of requiring an explicit module initialization
function to be called. An example use of static
initialization follows.

Note that the static initialization both eliminates the need for the
initialization test inside pthread_once() and the fetch of &foo_mutex
to learn the address to be passed
to pthread_mutex_lock() or pthread_mutex_unlock().

Thus, the C code written to initialize static objects is simpler on
all systems and is also faster on a large class of systems;
those where the (entire) synchronization object can be stored in application
memory.

Yet the locking performance question is likely to be raised for machines
that require mutexes to be allocated out of special
memory. Such machines actually have to have mutexes and possibly condition
variables contain pointers to the actual hardware locks.
For static initialization to work on such machines, pthread_mutex_lock()
also has to test whether or not the pointer to the actual lock has
been allocated. If it has not, pthread_mutex_lock() has to initialize
it before use. The reservation of such
resources can be made when the program is loaded, and hence return
codes have not been added to mutex locking and condition
variable waiting to indicate failure to complete initialization.

This runtime test in pthread_mutex_lock() would at first seem
to be
extra work; an extra test is required to see whether the pointer has
been initialized. On most machines this would actually be
implemented as a fetch of the pointer, testing the pointer against
zero, and then using the pointer if it has already been
initialized. While the test might seem to add extra work, the extra
effort of testing a register is usually negligible since no
extra memory references are actually done. As more and more machines
provide caches, the real expenses are memory references, not
instructions executed.

Alternatively, depending on the machine architecture, there are often
ways to eliminate all overhead in the most
important case: on the lock operations that occur after the
lock has been initialized. This can be done by shifting more
overhead to the less frequent operation: initialization. Since out-of-line
mutex allocation also means that an address has to be
dereferenced to find the actual lock, one technique that is widely
applicable is to have static initialization store a bogus value
for that address; in particular, an address that causes a machine
fault to occur. When such a fault occurs upon the first attempt
to lock such a mutex, validity checks can be done, and then the correct
address for the actual lock can be filled in. Subsequent
lock operations incur no extra overhead since they do not "fault".
This is merely one technique that can be used to support
static initialization, while not adversely affecting the performance
of lock acquisition. No doubt there are other techniques that
are highly machine-dependent.

The locking overhead for machines doing out-of-line mutex allocation
is thus similar for modules being implicitly initialized,
where it is improved for those doing mutex allocation entirely inline.
The inline case is thus made much faster, and the
out-of-line case is not significantly worse.

Besides the issue of locking performance for such machines, a concern
is raised that it is possible that threads would serialize
contending for initialization locks when attempting to finish initializing
statically allocated mutexes. (Such finishing would
typically involve taking an internal lock, allocating a structure,
storing a pointer to the structure in the mutex, and releasing
the internal lock.) First, many implementations would reduce such
serialization by hashing on the mutex address. Second, such
serialization can only occur a bounded number of times. In particular,
it can happen at most as many times as there are statically
allocated synchronization objects. Dynamically allocated objects would
still be initialized via pthread_mutex_init() or pthread_cond_init().

Finally, if none of the above optimization techniques for out-of-line
allocation yields sufficient performance for an
application on some implementation, the application can avoid static
initialization altogether by explicitly initializing all
synchronization objects with the corresponding pthread_*_init()
functions,
which are supported by all implementations. An implementation can
also document the tradeoffs and advise which initialization
technique is more efficient for that particular implementation.

Destroying Mutexes

A mutex can be destroyed immediately after it is unlocked. For example,
consider the following code:

In this case obj is reference counted and obj_done() is
called whenever a reference to the object is dropped.
Implementations are required to allow an object to be destroyed and
freed and potentially unmapped (for example, lines A and B)
immediately after the object is unlocked (line C).

FUTURE DIRECTIONS

None.

SEE ALSO

COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .